Mailbox unavailable. The server response was: 5.7.1 Unable to relay for [email protected]

I am getting “Mailbox unavailable. The server response was: 5.7.1 Unable to relay for [email protected]” when I try to send the mail using ASP.NET. The site is deployed on IIS7, Windows 2008 server. The website was working fine on IIS6 and Windows 2003 Server. I deployed it on IIS7, 2008 it has started giving me … Read more